The invention refers to the field of electronic reproduction technology and is directed to a device for clamping sheet-shaped recording material onto a recording drum with vacuum suctioning in an electronic reproduction device, and is also directed to a method for clamping. For example, the electronic reproduction device is a color scanner or a color recorder for producing color separations for multi-color printing.
In such an electronic reproduction device, sheet-shaped film material, also referred to as film proofs, are clamped onto the recording drum and fixed thereat by vacuum suctioning. The clamped film proofs are exposed point-by-point and line-by-line by a recording element, are then in turn unclamped from the recording drum and developed. The developed film proofs are the color separations for the multi-color print.
The film proofs can already be present in the form of sheet film material that is taken from a sheet film cassette for clamping or that is cut off before clamping from roll film material situated in a roll film cassette.
It is necessary for an effective employment of an electronic reproduction device, in particular, to shorten the preparation and setting times in comparison to the times required for the actual film exposure. This can occur, among other things, in that optimally many work steps normally to be executed by the operator such as, for example, the clamping and unclamping of the film proofs onto or, respectively, from the recording drum are automated.
Added thereto is the desire to be able to expose film proofs of different formats, particularly large-format film proofs, for example for reproducing posters.
DE-B-22 09 515 already discloses an electronic reproduction device having a device for clamping sheet-shaped recording material onto a recording drum with vacuum suctioning.
The recording drum is a hollow cylinder that is closed by covers. Shaft extensions with which the recording drum is rotatably seated at the device carrier of the reproduction device are secured to the covers. The walls of the hollow cylinder are provided with suction holes for suctioning the film proofs against the drum surface. The suction holes are in communication with the interior of the drum. One shaft extension is hollow, a suction channel thus arising that connects the interior of the drum to a stationary vacuum pump via a rotary transmission leadthrough secured to the shaft extension and via a stationary suction line.
The film proofs to be exposed, which are situated in a sheet film cassette, have their registration perforations hooked over registration pins situated on the recording drum on the basis of a manually actuatable roller and lever mechanism in the known reproduction device. The vacuum pump is then activated and the recording drum is turned, as a result whereof the film proofs are pulled from the sheet film cassette, are wound around the rotating recording drum and are fixed on the drum surface by the vacuum. After the exposure, the film proofs are unclamped and conveyed back into the roll film cassette by the roller and lever mechanism.
The apparatus disclosed by DE-B-22 09 515 has the disadvantage that only film proofs of given formats and provided with registration perforations can be clamped and that the clamping and unclamping can only ensue with the collaboration of an operator.
U.S. Pat. No. 4,268,841 discloses an electronic reproduction device that already comprises a means for the automatic clamping of sheet-shaped recording material onto a recording drum, for vacuum suctioning of the recording material against the recording drum and for the automatic unclamping of the exposed recording material from the recording drum. The sheet-shaped recording material is automatically conveyed from a supply station to the recording drum via a conveyor means, is wound around the recording drum with the assistance of an annular channel between drum surface and housing wall and is fixed thereon by vacuum suctioning. After the exposure, the recording material is likewise automatically unclamped from the recording drum and supplied to an exit opening in the device for further-processing. The generated surface of the recording drum is provided with suction holes arranged in rotational direction that are connected to a stationary vacuum pump via the interior of the recording drum and via a suction line in the shaft. A vacuum control means with which the vacuum for the suction holes can be switched on and off or set to an intermediate value is situated in the suction line.
The apparatus disclosed by U.S. Pat. No. 4,268,841 has the disadvantage that only sheet-shaped recording material can be clamped on and unclamped and that no specific measures for the reliable, fold-free clamping of recording material in different formats are recited that guarantee a smooth, automatic execution.
The devices disclosed by DE-B-22 09 515 and by U.S. Pat. No. 4268,841 have the further disadvantage that all suction holes of the recording drum are simultaneously charged with vacuum when the vacuum pump is switched on at the beginning of the clamping process. Particularly given small-format film proofs, high vacuum losses thereby arise since only a small number of suction holes are covered by the film proof.
As a result of the high vacuum losses, a reliable clamping of the film proofs is then not guaranteed during the exposure time wherein the recording drum rotates at high speed. The suction and, thus, the fixing of the film proofs can in fact be improved by installing a vacuum pump having a high nominal power; this, however, would be involved.
It is already known for reducing the vacuum losses to respectively seal the suction holes not covered by the momentarily clamped film proof by adhesive strips or to employ a recording drum wherein the non-covered suction holes are automatically closed with valves. Sealing the suction holes with adhesive strips is time-consuming and is involved and unreliable with automatically operating valves.
It is likewise already known to subdivide the recording drum by partitions into individual, connectable vacuum chambers. In this case, a reduction of the vacuum losses can be achieved in that respectively only those vacuum chambers are activated that, based on the axial format length, momentarily participate in the fixing of the film proofs.
DE-A-32 30 676 already discloses a vacuum clamping means that employs a combination of pneumatic and mechanical components for fixing sheet-shaped recording material on a recording drum. The suction holes of the recording drum are combined in at least two groups, each of which comprises one or more rows of suction holes connected to one another. The individual suction hole groups are in communication with one another by throttle lines. At least one group is connected to a stationary vacuum generator.
What is achieved by adding individual suction hole groups or suction hole rows is that recording material having different formats can be clamped without high vacuum losses. Means for the automatic control of the valves and for conveying the recording material to or from the recording drum are not specified.
The apparatus disclosed by DE-A-32 30 676 thus has the disadvantage that the feed of the sheet-shaped recording material must ensue manually and that the switching of the suction hole groups during the clamping or unclamping event does not ensue automatically dependent on the rotational motion of the recording drum or, respectively, on the respective wrap of the recording material around the recording drum and, thus, dependent on the respective format of the recording material to be clamped on.
The tendency in reproduction technology is to employ unperforated roll film material and to cut the respectively required film lengths from the roll film material and clamp them on. Added thereto is the desire to be able to clamp and release arbitrary formats. Practice has shown that it is difficult to clamp, in particular, large-format film proofs onto the recording drum exactly and fold-free. An exact and smooth clamping of the film proofs, however, is an indispensable prerequisite for the exposure of exactly registered color separations and for the production of qualitatively high-grade multi-color prints.